The effect of dietary protein restriction on the secretion of LH and FSH in pre-pubertal female lambs

The effect of restricted dietary protein on the synthesis, storage and release of LH and FSH was studied in pre-pubertal female lambs. The experiment started when the lambs were aged 12 weeks and weighed 26.0+/-1.6 kg. It was conducted for 25 weeks. The lambs were fed isocaloric diets containing either a restricted level of crude protein (8% CP; n=6; treatment R) or an elevated one (18% CP; n=4; treatment E). At 37 weeks of age and before the first oestrous cycle, blood samples were collected over 6 h at 10 min intervals for LH assay. The lambs were slaughtered and their brains recovered and fixed in situ. Immuno-reactive (IR) LH and FSH cells were localised by immunohistochemistry techniques. Messenger RNA analyses used by non-isotope in situ hybridisation with sense and anti-sense riboprobes from beta subunits of LH and FSH cDNA clones. Data were generated using computer analysis to measure the proportion of IR and/or hybridising cells and their optical density for immuno-staining and hybridisation signal. Plasma LH was measured by RIA. The daily live-weight gains were 56.5+/-13.1 g and 97.8+/-14.3 g for R and E lambs, respectively (P<0.05), so that final weights at slaughter were 36.1+/-1.97 kg and 39.1+/-3.44 kg, respectively (P<0.05). The number of cells expressing LHbeta mRNA and the optical density of this hybridisation signal was significantly (P<0.001) lower in the R lambs but the number of IR LH positive cells was higher (P<0.001) than for the E lambs. The concentration of LH in the plasma of R sheep was lower (P<0.05) than the E group and this response was associated with a decrease (P<0.05) in LH pulse frequency and amplitude. Dietary protein concentration appeared to have no effect on the IR in FSH cells or on the expression of FSHbeta mRNA. In summary, the low protein diet influenced the body weight and weight gain of growing lambs and exerted an inhibitory effect on the synthesis and the release of LH in the pituitary gonadotrophs. No such effect was observed for FSH. It was concluded that the protein concentration of the diet consumed during the growth of female lambs may be an important modulator of processes leading to the pre-pubertal rise in LH.